Effect of selected soil water conservation practices on water budgeting in Robusta coffee (coffea canephora) in Uganda

Abstract

Coffee is a vital crop globally, employing over 125 million people. In Uganda, more than 12 million depend on coffee-related activities, yet yields remain below the potential of 2.2 tons/ha due to pests, diseases, poor management, and climate change. Climate change, marked by erratic rainfall and rising temperatures, increases evapotranspiration and crop water stress. While studies on Arabica dominate, there is limited information on how soil moisture and conservation practices affect Robusta coffee production in Uganda. This study addressed three objectives. First, the effect of climate change on soil moisture was assessed using historical data (1990-2022) and projections (2025-2050) from eight Global Climate Models under SSP245 and SSP585 scenarios. Soil moisture suitability maps were generated using MATLAB climatology tools, Mann-Kendall trend analysis, and a Mamdani Fuzzy Inference System. Findings revealed that historically, soil moisture was highest around Lake Victoria (115-143 mm) and lowest in Northern and Southwestern Uganda (<30 mm). SSP245 projections indicate a slight moisture decrease near Lake Victoria but an increase in other regions, with Rwenzori and Kitgum seeing the highest rise (0.08 mm). Under SSP585, soil moisture remains high in Kigezi and Rwenzori with varying yearly patterns. Historically, 71% of Uganda was highly suitable for coffee, but future predictions suggest suitability will rise to 74% under SSP245 and 81% under SSP585, potentially boosting coffee suitability by 10%. Second, water use efficiency (WUE) of Robusta coffee under Albizia coriaria shade (ACS) and open sun (COSS) systems was evaluated at the National Coffee Research Institute (NaCORI) using a randomized block design. Sap flow meters measured water use, while growth, yield, and phenological data were collected. Coffee in the open sun significantly (p<0.001) used more water per day (1.43 l/day) compared to coffee under shade (0.62 l/day). There was a significant difference (p<0.001) in water use per unit leaf area per system. ACS used more water (0.12 mm/day) compared to COSS (0.10 mm/day). WUE varied by system and phenological stage, with ACS being more efficient during fruit development stages. Third, the effect of soil moisture conservation practices (SMCPs) on soil-water relations was assessed at NaCORI and Kaweri Coffee Plantation Limited (KCPL). Four systems were tested: COSS, coffee cover crop (CCS), coffee mulch (CMS), and ACS. CMS significantly (p<0.05) increased soil moisture by 45.32% at NaCORI, followed by CCS at 13.68%, while ACS decreased soil moisture by 29.32% within the coffee root zone. At KCPL, there was no significant difference in the efficiency of SMCPs in conserving soil moisture, though, CCS increased soil moisture by 21.29%, followed by ACS (20.17%), with the least increase observed under CMS (16.17%). In conclusion, Climate change will result in higher soil moisture, which will increase the suitability of coffee by 10%. Some areas that are currently suitable like, Northern Uganda will not be suitable in the future. CMS, CCS and ACS improved the soil water relations better compared to COSS. Therefore, near future unstable areas should adopt soil moisture conservation practices to reduce on water stress likely to come due to climate change. Albizia coriaria should be optimally used as a way of improving water use efficiency for better fruit development without compromising on bean weight and yield.